The mechanisms of myocardial cell injury during ischemia and reperfusion are not well understood. An increase in intracellular free ionized calcium concentration, [Ca2+]i, is thought to be involved although the relative contributions of various Ca2+ flux pathways and release from intracellular Ca2+ stores have not been defined. The specific aims of this research are: 1) to define changes in [Ca2+]i and [ATP] during contracture produced by metabolic inhibition of ATP production; 2) to define the effects of inhibitors of Na-Ca exchange, slow Ca2+ channel blocking drugs, and an Alpha adrenergic antagonist on contracture, [ATP], and [Ca2+]i under these conditions; 3) to investigate the effects of these drugs on Ca2+ fluxes, and Ca2+ contents, [Ca2+]i, [ATP] and contraction and relaxation following abrupt release of metabolic inhibition of ATP production; 4) to determine the effects of treatment with these drugs before and after metabolic inhibition on the activity of the Na-K ATPase Na pump, membrane potential, and cellular contents of Na and K; 5) to determine the effects of these drugs on myocardial cell injury during and after abrupt release of metabolic inhibition. The overall goal of these studies is to understand the mechanisms which contribute to calcium overload during ATP depletion, and the extent to which drugs that alter Ca fluxes have a beneficial effect on ischemic dysfunction and injury. In these experiments we will use cultured monolayers of chick embryo ventricular cells in which unidirectional transmembrane fluxes and cellular contents of Ca, K, and Na may be measured using radioactive isotopes. [Ca2+]i will be measuered optically using Quin2 and/or Arsenazo III. Ischemia will be partially simulated by inhibition of oxidative phosphorylation with cyanide or hypoxia, and of glycolysis with 2-deoxyglocuse. The results of these studies will be relevant to pharmacologic prevention and treatment of ischemic myocardial injury, particularly reperfusion injury after streptokinase induced lysis of coronary artery thrombosis, or bypass graft surgery for acute myocardial infarction; and during prolonged ischemic cardioplegia for cardiac surgical procedures. Furthermore, these studies should increase our general understanding of the ways in which myocardial cell calcium homeostasis is disturbed during ischemia and reperfusion.